Saudi Arabia launched air strikes in Yemen on Thursday in coalition with Gulf region allies to counter Iran-backed forces besieging the southern city of Aden, where the U.S.-supported Yemeni president had taken refuge.

According to Al Arabiya TV, Saudi Arabia is contributing 100 warplanes and 150,000 soldiers to the Yemen operation. Egypt, Pakistan, Jordan, Sudan are ready to participate in a ground offensive.

Yemeni Foreign Minister Riyadh Yaseen said Saudi-led air strikes launched on Thursday were targeting Houthi air power and the militia’s ability to launch missiles.

Yaseen said among the targets were the Dulaimi air base, Taiz air base and Hodeidah air base “because they have been taken over by the Houthis.”

The White House said President Barack Obama has authorized logistical and intelligence support to the Saudi-led operation, adding that U.S. forces will not be taking a direct part in the operations.

There were indications that others in the region would follow suit: The United Arab Emirates, Kuwait, Qatar and Bahrain joined Saudi Arabia in a statement published by the Saudi Press Agency, saying they would answer a request from Hadi “to protect Yemen and his dear people from the aggression of the Houthi militias which were and are still a tool in the hands of foreign powers that don’t stop meddling with the security and stability of brotherly Yemen.” Oman, the sixth member of the Gulf Cooperation Council, didn’t sign onto the statement.

In a statement from the state news agency Egypt, too, announced political and military support. “There is coordination ongoing now with Saudi Arabia and the brotherly gulf countries about preparations to participate with an Egyptian air and naval forces and ground troops if necessary,” the statement said.

Saudi Ambassador to Washington Adel al-Jubeir told reporters a 10-country coalition had joined in the military campaign in a bid “to protect and defend the legitimate government” of Yemen President Abd-Rabbu Mansour Hadi.

Jubeir declined to give any information on Yemen’s President Hadi’s whereabouts, but said the president, who has fled his residence, was still running the government along with members of his Cabinet.

Jubeir said Iranian-backed Houthi Shi’ite militants were now in control of the Yemeni air force and of the country’s ballistic weapons.

“This is a very dangerous situation and we must do everything we can to protect the people of Yemen and protect the legitimate government of Yemen,” Jubeir told a news conference at the embassy.

But he declined to give any information on the whereabouts of Hadi, though he said the military action was being taken at the embattled U.S.-backed leader’s direct request. The United States said earlier that Hadi, holed up in Aden since fleeing the Houthi-controlled capital Sanaa last month, was no longer at the compound he has been using as a base.

He said the mission would not be limited to a specific city or region of Yemen, suggesting that the coalition’s warplanes could strike the Houthis anywhere they choose.

Jubeir said the operation, which was launched at 7 p.m. EDT (2300 GMT) on Wednesday in response to a request for assistance by Hadi, was not limited to one particular city or region.

“We have air assets from a number of countries in the (Saudi) kingdom and we have military assets that are on their way to the kingdom to participate in these operations,” Jubeir said.

Meanwhile, warplanes launched an attack on Sanaa airport and the Yemeni capital’s al Dulaimi military airbase, residents and an official said. There was no immediate word on the affiliation of the aircraft.

The Houthi-run al-Masirah television reported the airstrikes targeted a residential neighborhood north of the capital Sanaa, causing dozens of casualties. The satellite channel also urged medical personnel to urgently report to hospitals in the capital.

A senior leader of Yemen’s Houthi movement said on Thursday that Saudi air strikes amounted to an aggression against the country and warned they would set off a “wide war” in the region.

“There is an aggression underway on Yemen and we will confront it valiantly,” Mohammed al-Bukhaiti, a member of the Houthis politburo, told the Doha-based al Jazeera television.

“Military operations will drag the region to a wide war.”

Houthi militia forces and allied army units seized Aden airport and a nearby air base on Wednesday, tightening their grip on the outskirts of the southern Yemeni city.

Local officials said troops loyal to Yemen’s ex-President Ali Abdullah Saleh, a powerful ally of the Houthis, had captured Aden airport in late afternoon but that clashes with Hadi supporters were continuing in the vicinity. The airport was closed and all flights were cancelled.

Earlier the Houthis and their allies took al-Anad air base 60 km (37 miles) north of the city before continuing their southward advance.

Yemen’s slide toward civil war has made the country a crucial front in mostly Sunni Saudi Arabia’s rivalry with Shi’ite Iran, which Riyadh accuses of stirring up sectarian strife through its support for the Houthis.

Sunni Arab monarchies around the region have condemned the Shi’ite Houthi takeover as a coup and have mooted a military intervention in favour of Hadi in recent days.

U.S. officials said earlier that Saudi Arabia was moving heavy military equipment including artillery to areas near its border with Yemen. But Saudi sources said earlier on Wednesday that the build-up, which also included tanks, was purely defensive.

Meanwhile, brent crude oil prices rose by more than a dollar in early Asian trading on Thursday after the military operation in Yemen began. Brent crude oil futures LCOc1 rose to 57.95 a barrel at 0215, up almost $1.50 since their last settlement. U.S. crude CLc1 was up $1.20 at $50.41 a barrel.

A widening Yemen conflict could pose risks for global oil supplies. Most oil tankers from Arab producers like Saudi Arabia, the United Arab Emirates, Kuwait and Iraq have to pass Yemen’s coastlines via the tight Gulf of Aden in order to get through the Red Sea and Suez Canal to Europe.